I upgraded to version 2.5. After that the shadow skin names of the months, tyhe eventslist header, weeknames in de cvalendar are english, after reinstaling the dutch laguage file the menu's and so on are dutch again. Tried to instal the Shadow4-laguages.r2skin-addon but it said that the shadow4-laguage skin does not exist. After renaming the file to Shadow4.r2skin-addon it installed but still all the names are in english.
Is Dutch in there? Or do I do something wrong?

The old versions of the addon don't have version info. The newer ones only install over a specific version of the skin. If you have the addon already installed you need to revert back to the original skin before it can be installed again. You can do that e.g. by reinstalling the application (reinstallation doesn't overwrite your settings or events).